Various types of bandages and wound dressings are known and used to protect wounds and burns. Typically, wound dressings are fabricated with an absorbent material so that wound exudate is removed and the wound dried, facilitating healing. Wound dressings may also contain one or more pharmacologically active agents such as antibiotics, local anesthetics, or the like. Commonly used wound dressings include fibrous materials such as gauze and cotton pads, which are advantageous in that they are absorbent but problematic in that fibers may adhere to the wound or newly forming tissue, causing wound injury upon removal. Other wound dressings have been prepared with foams and sponges, but the absorbance of these materials is often limited. Furthermore, such wound dressings require the use of adhesive tape, as they are not themselves adhesive.
Hydrophilic pressure-sensitive adhesives (“PSAs”) are used in a variety of pharmaceutical and cosmetic products, such as topical and transdermal drug delivery systems, wound dressings, face masks, bioadhesive films designed for buccal and mucosal administration, teeth whitening strips, and so on. A general distinctive feature of hydrophilic PSAs is that they typically adhere to wet biological substrates, while conventional hydrophobic (rubber-based) PSAs typically lose their adhesive properties when moistened.
The adhesive properties of PSAs will vary depending upon how and where the products are to be used. For transdermal drug delivery and topical applications, an adhesive patch, for instance, should provide high tack immediately upon use, and such tack should be maintained during the entire application period (from one day to one week). For buccal patches and teeth strips, it is often desirable to use elastic polymer films, which exhibit no adhesion towards dry surfaces, but are highly tacky when applied to hydrated, soft mucosal surfaces and/or moistened solid tissue surfaces such as teeth. For wound dressings and other various purposes, in order to avoid skin damage upon patch removal, either water-soluble adhesives or insoluble hydrogel adhesives, which lose their adhesion under swelling in a large amount of water, are preferred. Face masks and some tooth whitening products best utilize hydrophilic polymer compositions in the form of aqueous or ethanol-water solutions, which become dry after placement on a surface, thereby forming an insoluble, polymer film that adheres to the underlying tissue surface, but does not adhere to other surfaces.
In order to effectively tailor the adhesive properties of polymer materials useful in pharmaceutical and cosmetic products, a design method has been developed based on molecular insight into mechanisms underlying the adhesive properties. As has been recently established, at a molecular level, the pressure-sensitive adhesion is due to coupling of two apparently incompatible types of molecular structures. This reveals that there is a fine balance between strong cohesive interaction energy and enhanced free volume. See, for example, Feldstein et al. (1999) Polym. Mater. Sci. Eng., 81:465-466; Feldstein et al., General approach to the molecular design of hydrophilic pressure-sensitive adhesives, Proceed. 25th Annual Meeting Adhesion Soc. and 2nd World Congress on Adhesion and Relative Phenomena, February 2002, Orlando, Fla., vol. 1 (Oral Presentations), p. 292-294; and Chalykh et al. (2002) J. Adhesion 78(8):667-694.
The “free volume” property of the molecular structure of PSA polymers results in high tack at a macroscopic level and a liquid-like fluidity of the PSA material, which allows for a fast-forming adhesive bond. The “cohesive interaction energy” or “cohesion energy” property defines the cohesive toughness of the PSA polymer and provides the dissipation of detaching energy in the course of adhesive joint failure. Based on this finding, a general method for obtaining novel hydrophilic adhesives is described in U.S. Pat. No. 6,576,712 to Feldstein et al., which involves physically mixing non-adhesive, hydrophilic, high-molecular-weight polymers with appropriate short-chain plasticizers.
In various PSAs, different molecular structures provide proper amounts of cohesion energy and free volume, thereby defining the adhesive properties of the polymer materials. For instance, in acrylic PSAs, strong cohesive interaction energy is a result of mutual hydrophobic attraction of the alkyl radicals in side chains, whereas large free volume is due to either electrostatic repulsion of negatively charged carboxyl groups or a large volume of isoalkyl radicals in the side chains. In synthetic rubbers, a large free volume is obtained by adding high volume, low-density molecules of tackifying resins. In hydrophilic adhesives, when high molecular weight polyvinyl lactams (i.e. poly(N-vinyl-2-pyrrolidone) (“PVP”) or polyvinyl caprolactame (“PVCap”)) are blended with the short-chain polyethylene glycol (“PEG”), as described in U.S. Pat. No. 6,576,712, high cohesive strength results from hydrogen bonding between, for example, PVP carbonyl groups and complementary terminal hydroxyls of PEG, while the large free volume is due to the location of reactive groups at both ends of the PEG chains, which are of appreciable length and flexibility.
A proper balance between high cohesion energy and large free volume, which is responsible for adhesive properties of polymer materials, is achieved by evaluating the various PSA properties. For instance, the ratio between cohesion energy and free volume defines the value of glass transition temperature, Tg, and elasticity modulus, E, of a polymer. Higher cohesion energy and lower free volume, results in higher values for both Tg and E. It is well recognized that all PSAs demonstrate a Tg in the range of about −55 to −30° C. and an E≈1−105 Pa.
In U.S. Pat. No. 6,576,712, the hydrophilic polymers and plasticizer are capable of hydrogen bonding or electrostatic bonding to each other and are present in a ratio that optimizes key characteristics of the adhesive composition, such as adhesive strength, cohesive strength and hydrophilicity. The plasticizer has complementary reactive functional groups at both ends and when both terminal groups interact with complementary functional groups in the hydrophilic polymer, the plasticizer acts as a non-covalent crosslinker between the longer chains of hydrophilic polymer. In doing so, the plasticizer combines the plasticization effect with enhanced cohesive toughness of the PSA polymer blend. This molecular design method for tailoring new hydrophilic PSAs describes the adhesive capability of long-chain, high Tg hydrophilic polymers, as well as the ratio of hydrophilic polymer to plasticizer (cohesive enhancer), which provides the best adhesion.
When dry, the adhesives described in U.S. Pat. No. 6,576,712, e.g. the blends of high molecular weight PVP with oligomeric PEG ranging in molecular weight from 200 to 600 g/mol, provide rather low adhesion toward dry surfaces. Adhesion increases when the surface of a substrate is moistened or the adhesive absorbs water. The maximum adhesion is observed when the adhesive contains 5-10% of absorbed water. This is usually the case when the adhesive is exposed to an atmosphere having 50% relative humidity. Additionally, under direct contact with water, the adhesive dissolves. However, these adhesives not contain covalent crosslinks, and are thus not suitable for applications that require swellable yet water-insoluble adhesives. In particular, these prior art adhesives are less useful when increased adhesion is desired upon much more appreciable hydration levels (e.g., 15% of absorbed water and higher).
Therefore, while the prior art discloses polymers and hydrogel compositions that can be tailored with respect to cohesive strength, adhesive strength, tack, elasticity, and water swellability, it remains desirable to develop a molecular design method for preparing novel hydrophilic PSAs that focuses on balancing cohesive interaction energy and free volume at a molecular level.
In order to resolve these problems, this invention is directed to a method of obtaining water-insoluble, film-forming compositions by blending soluble polymers. While this has been attempted in the past, e.g., U.S. Pat. No. 5,597,873 to Chambers et al. and U.S. Pat. No. 5,306,504 to Lorenz et al. (mixing carboxyl-containing polymers with polyhydric alcohols and polyamines) and U.S. Pat. No. 4,771,105 to Shirai et al. and U.S. Pat. No. 5,726,250 to Zajaczkowski (crosslinking of polyacrylic acid “PAA” or the copolymers of acrylic acid with the salts of di- and trivalent metals (Ca2+, Al3+), all of these procedures are directed to the production of non-adhesive water absorbents by mixing techniques.